1. Field of the Invention
The present invention relates to a piezoelectric oscillator used as a frequency controlling device or the like, and in particular to a thin and highly stable piezoelectric oscillator configured such that a piezoelectric vibrator is heated by a heater and the temperature of the heater is controlled by a temperature controlling circuit, where thinning to an almost extreme extent can be achieved without using such an oven as a metal block and various requirements including stability of an oscillation frequency, a low power consumption, improvement in assembling easiness and cost reduction are simultaneously satisfied.
2. Description of the Related Art
As such a piezoelectric oscillator as a quartz oscillator which is a frequency controlling device used in mobile communication equipment or transmission communication equipment, an oven type piezoelectric oscillator which can output a highly stable frequency without being affected by an external temperature change has been conventionally known. Recently, in these technical fields, since each equipment is required to be reduced in size and weight and be portable, the oven type piezoelectric oscillator is also required to be reduced in size and weight from the market correspondingly. That is, the height of the conventional oven type piezoelectric oscillator is usually 20 mm or more. In some cases, an oven type piezoelectric oscillator as big as up to about 50 mm in height is frequently used. In recent years, however, it is strongly required to reduce the oven type piezoelectric oscillator down to a height equal to that of a large-sized part so as to correspond to miniaturization of each equipment. At present, the thinnest one of oven type piezoelectric oscillators which have been developed by the inventor of the present application has a height of 9.2 mm, but an even more thinner oven type piezoelectric oscillator is required from the market. The conventional oven type piezoelectric oscillator is configured for obtaining a highly stable frequency such that a piezoelectric vibrator is received in a recess portion of such an oven as metal block having a large heat capacity and the metal block is heated to a predetermined temperature by a heater. However, since the bulk of the whole oscillator increases by using a metal block with a large size, the requirement such as reduction in size or weight can not be satisfied. Further, since such a configuration is employed that a piezoelectric vibrating element inside a piezoelectric vibrator is heated via the metal block, there is a problem that it takes time until heat from the heater is conducted to the piezoelectric vibrating element so that a desired frequency is obtained.
For example, a conventional oven type piezoelectric oscillator of a surface mounting type (disclosed in FIG. 5 of Japanese Laid-open application publication No. 2001-274626) has such a configuration that such a piezoelectric vibrator as a quartz vibrator and a printed board on an oscillator side are received sideways in a metal block (oven) and a heater wire is wound on an outer face of the metal block. The metal block is fixed on a mother printed board mounted with oscillating circuit parts on a top face through a predetermined gap, and the metal block and the mother printed board are received in a metal case. The metal case is mounted on a printed board on an equipment main body side by utilizing lead members projecting from a bottom portion of the metal case. Current is supplied to the circuit parts from the lead members via a wiring pattern on the mother printed board and current is supplied to the piezoelectric vibrator or the heater wire. However, in the oven type piezoelectric oscillator of this type, since the height of the metal block itself to be used is large and the metal block is fixed in a floating manner from the top face of the mother printed board, the thickness of the main body of the oven type piezoelectric oscillator becomes considerably large as compared with the thickness of the piezoelectric vibrator. Therefore, the height of the whole oscillator becomes large.
Next, as an oven type piezoelectric oscillator which has been further thinned without using a heater wire (disclosed in FIG. 6 of the Japanese Laid-open application publication No. 2001-274626) such an oscillator that a piezoelectric vibrator is disposed in a lying state so as to contact with a heater resistor mounted on a top face of a mother printed board mounted with circuit parts on its lower face, the piezoelectric vibrator is covered with a metal block whose lower face is opened toward the piezoelectric vibrator and these constituent elements are enclosed in a metal case has been proposed. The oven type piezoelectric oscillator of this type can be configured such that the thickness of the piezoelectric vibrator approximates roughly to the thickness of the main body of the oven type piezoelectric oscillator, but the limit of the height obtained by such a configuration will be 9.2 mm at the most, and further reduction in height will be impossible. A thick metal block made of aluminum alloy or the like is still required in order to heat the piezoelectric vibrator, which results in a factor for inhibiting size and height reduction. Further, since oscillating circuit parts are mounted on a lower face of the mother printed board, some of the oscillating circuit parts are not only separated from the heater but also they approach to a bottom base of the metal case. Therefore, temperature change of the oscillating circuit parts becomes large due to increase in heat dissipation which results in adverse affection on temperature characteristics of the oscillator. In order to stabilize the temperature characteristics of the oscillator, though it is important to stabilize not only the temperature of the piezoelectric vibrator but also the temperature of the oscillating circuit parts, the oven type piezoelectric oscillator does not have a configuration satisfying such a requirement.
In order to solve these problems to achieve further thinning, the present inventor have proposed, in the Japanese Laid-open application publication No. 2001-274626, a surface mounting type thin and highly stable piezoelectric oscillator in which a piezoelectric vibrator is fitted into a notch formed in a mother printed board to achieve thinning corresponding to the thickness of the mother printed board, these constituent elements are respectively enclosed from both surface and back sides by a flexible printed board, heater resistors mounted on the flexible printed board are brought in close contact with both the surface and back of the piezoelectric vibrator, and these constituent elements are enclosed in a case. The piezoelectric oscillator has achieved thinning up to 6.2 mm successfully.
However, in the piezoelectric oscillator described above, since the heater resistors mounted on the flexible printed board are brought in close contact with both the surface and back of the piezoelectric oscillator, the oscillating circuit parts mounted on the mother printed board and the heater resistors take a positional relationship where they are separated from each other, which interferes with stability of a heating temperature of the oscillating circuit parts. Further, since a capacitor and the like serving as adjusting circuit parts are mounted on the mother printed board whose surface and back surface have been blocked by the flexible printed board, it becomes impossible to adjust the adjusting circuit parts after assembling has been completed.
It is an object of the present invention to provide a thin and highly stable piezoelectric oscillator configured such that a temperature of a heater is controlled by a temperature control circuit, where thinning to an almost extreme extent can be achieved without using such an oven as a metal block causing size-enlargement and various requirements including stability of an oscillation frequency, a low power consumption, improvement in assembling easiness and cost reduction are simultaneously satisfied.
Further, another object of the present invention is to provide a thin and highly stable piezoelectric oscillator where characteristics are stabilized by heating not only a piezoelectric vibrator but also oscillating circuit parts by a heater resistor sufficiently, and adjusting work can be implemented arbitrarily by exposing adjusting circuit parts externally.